This invention relates generally to techniques for thin film deposition, and more particularly, to techniques for stabilizing precursors to produce thin films.
Thin film deposition, using MOCVD (metal organic chemical vapor deposition), is an important fabrication method for a variety of electrical and electro-optical materials, for example, super conductor materials and optical wave guides. Some MOCVD reactors sublime one or more solid precursors and transport the resulting vapor in an inert carrier gas to a heated substrate upon which certain chemical reactions occur resulting in the deposition of a thin film product.
Lithium Niobate (LiNbO.sub.3) is an electro-optic ceramic which may be prepared using one of a variety of methods, including MOCVD, as described by Wernberg, A. A., and Gysling, H. J., MOCVD Deposition of Epitaxial LiNbO.sub.3 Thin Films Using the Single-Source Precursor LiNb(O Et).sub.6. The reference lists numerous manufacturing methods for LiNbO.sub.3 including MOCVD methods. For example, LiNbO.sub.3 has hitherto been manufactured in a two-source MOCVD process using lithium 2,2,6,6-tetramethylheptane-3,5-dionate (Li(thd)) and Nb(OMe).sub.5. MOCVD fabrication of LiNbO.sub.3 is also described in U.S. Pat. No. 3,911,176, to Curtis et al., entitled "Method for Vapor-phase Growth of Thin Films of Lithium Niobate" wherein a substrate of Lithium tantalate (LiTaO.sub.3) is coated with a thin film of LiNbO.sub.3 obtained from Li(thd) and Nb(OMe).sub.5 precursors.
Additionally, LiNbO.sub.3 has been manufactured by a MOCVD process using a toluene solution of a single-source precursor generated in situ by the reaction of Nb(OEt).sub.5 and Li(thd).
A third method of manufacturing LiNbO.sub.3, reported by Wernberg and Gysling, uses a single-source reagent, LiNb(OEt).sub.6, in a spray MOCVD process.
A fourth method of manufacturing LiNbO.sub.3 is a two-source solid precursor MOCVD process using Li(thd) and Nb(thd).sub.4 precursors.
A number of difficulties exist with these methods of manufacturing LiNbO.sub.3. In the case of the first method, the method produces a film which is polycrystaline and black, therefore unsuitable for optical applications; the second method produces a rough surface; the third method have a certain amount of defects in the thin film surface due to the gas phase decomposition of the precursor; and the fourth method suffers from the premature oxidation of the Li(thd). This premature oxidation produces a lithium carbonate soot which is deposited into the thin film, thereby degrading the optical quality as well as reducing the growth rate of the film. Furthermore, it has been found that this method requires a 7-to-3 ratio of Li(thd) to Nb(thd) although the stoichiometry leads to a 1-to-1 expected ratio.
Two types of precursor thermal stability are of concern in MOCVD reactions. The first kind is the thermal stability of the precursor as it is being sublimed and transported in the carrier gas to the substrate. The second kind of thermal stability is the precursor stability in the very hot vicinity of the substrate.
The substrate in a MOCVD reactor for producing LiNbO.sub.3 is approximately 700.degree. C. Li(thd) is thermally stable to about 400.degree. C. The lithium carbonate soot is produced by oxidation of a certain amount of the Li(thd) in the hot vicinity of the substrate. The lithium carbonate is then deposited onto the substrate and is absorbed into the thin film.
A prior approach to solving this problem is to use commercially available Li(thd) prepared by the method described in Hammond, G. S, Nonhebel, D. C., and Wu, C-W. S., Inorg. Chem., 2, 73 (1963). The quantity of soot is minimized by reducing the deposition temperature of the film onto the substrate, thereby reducing the overall reactor temperature. However, there is nevertheless an unacceptable quantity of soot absorbed into the LiNbO.sub.3 film. Furthermore, the low deposition temperatures result in films with less than optimum crystalline properties.
In light of the foregoing, it is desirable to have a solid source MOCVD process for producing a high quality thin film LiNbO.sub.3 with no defects, no soot particles, and a required ratio of precursor quantities which approximates the stoichiometrically expected ratio.
There has been certain attempts in the prior art to chemically stabilize the precursors used in various MOCVD reactions. However, these prior art attempts focus on the first kind of precursor stability (i.e., stability during sublimation).
A number of stabilized complexes of the precursor for alkaline earth compounds used in the chemical vapor deposition of certain superconducting oxides, such as Y-Ba-Cu, Bi-Sr-Ca-Cu and Tl-Ba-Ca-Cu oxides are described in Timmer, K., and Meinema, H. A., Synthesis and characterization of BaX.sub.2 18-crown-6 complexes. Barium bis(1,1,1,5,5,5-hexafluoropentane-2,4-dionate)18-crown-6, a non-hygroscopic, thermally stable, volatile barium compound, Inorg. Chem. Acta, Vol. 187, pp. 99-106 (1991). Therein is described, for example, the preparation of Ba(thd)-18-crown-6. In the reference it was noted that Barium-.beta.-diketonates, used as precursors for the synthesis of thin films of YBa.sub.2 Cu.sub.3 O.sub.7-x, are prone to oligomerization and decomposition at the temperatures required to obtain the volatility necessary for MOCVD processes. However, the reference notes that complexes between Barium hexafluoropentane dionate including an 18-crown-6 ligand are volatile at lower temperatures and thermally stable at those temperatures. In Norman, J. A. T., and Pez, G. P., "Volatile Barium, Strontium and Calcium Bis(hexafluoroacetylacetenate)(crown ether) Complexes", J. Chem. Soc., Chem. Commun., pp. 971-972 (1991) similar properties are observed, also for Alkaline earth di-ketonate complexes with crown-ethers. However, the references do not discuss the thermal properties of the precursor near the substrate and do not discuss stabilization of non-alkaline earth compounds used in MOCVD reactions.
An additional problem found when using unstable precursors is the deposition of material on the reactor walls upstream from the substrate.
Accordingly, it would be desirable to stabilize the precursors for the production of alkali metal-containing oxides to avoid the premature oxidation, gas phase nucleation, and decomposition of the precursors, and thereby improve the quality of the thin films produced by MOCVD processes.